A two dimensional/three dimensional steerable catheter can deflect four pull cables, generating curves in four different planes. Currently, this is accomplished by having a distal tip segment that can be deflected into four independent quadrants using separate pull cables attached to a distal tip. The four pull cables are controlled by two independent mechanisms. One of the mechanisms consists of a thumb wheel that, when rotated both in a clockwise and a counterclockwise direction from a neutral position, generates tension to two of the four independent cables. The second mechanism utilizes a slider control, which when slid forward and backward from a neutral position, generates tension to the remaining two independent cables. The construction and operation of the thumb wheel mechanism is further described in U.S. Pat. No. 5,611,777 to Bowden, Russel W., Falwell, Gary S. et al, issued Mar. 18, 1997 and U.S. Pat. No. 5,904,667 to Falwell, issued May 18, 1999, each of these patents is hereby incorporated by reference in its entirety. After the user actuates either of the handle mechanisms, it is desired that the degree of deflection be maintained until actively changed by the user. Therefore, each of the mechanisms must generate a frictional holding force which is larger than the unloading force of a fully actuated curve.
However, the frictional holding force has been found to significantly decrease after the materials which make up the mechanism are exposed to 65° C. during the sterilization cycle. The reason for this degradation in holding force is that residual stresses are introduced during the assembly of the handle. These stresses appear to be close to the yield strength of the materials used within the assembly at ambient temperatures. When the materials are exposed to an elevated temperature, they experience a decrease in modulus, causing the materials to yield within the assembly resulting in a decrease in holding force.